The climate change debate has been going back and forth between skeptics and believers for the last couple of years. While carbon dioxide is usually the greenhouse gas that gets the most attention, methane is considered another powerful greenhouse gas that can be emitted both naturally as well as human-induced.

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A new study suggests the increase in methane emissions since the industrial revolution cannot be blamed on anthropogenic sources alone.

Lead author Logan Mitchell, who coordinated the research as a doctoral student at Oregon State University, said the "early anthropogenic hypothesis," which spawned hundreds of scientific papers as well as books, cannot fully explain on its own the rising levels of atmospheric methane during the past 5,000 years. That theory suggests that human activities such as rice agriculture were responsible for the increasing methane concentrations.

Opponents of that theory argue that human activities during that time did not produce significant amounts of methane and thus natural emissions were the dominant cause for the rise in atmospheric methane (CH4).

Mitchell, who is now a post-doctoral researcher at the University of Utah explains, "the increase in methane emissions during the late Holocene came primarily from the tropics, with some contribution from the extratropical Northern Hemisphere."

Scientists determine methane levels by examining ice cores from polar regions. Gas bubbles containing ancient air trapped within the ice can be analyzed and correlated with chronological data to determine methane levels on a multidecadal scale. Mitchell and his colleagues examined ice cores from the West Antarctic Ice Sheet Divide and the Greenland Ice Sheet Project and found differences between the two.

Ice cores from Greenland had higher methane levels than those from Antarctica because there are greater methane emissions in the Northern Hemisphere. The difference in methane levels between the hemispheres, called the Inter-Polar Difference, did not change appreciably over time.

"If the methane increase was solely natural or solely anthropogenic, it likely would have tilted the Inter-Polar Difference out of its pattern of relative stability over time," Mitchell said.

Mitchell used previous models that hypothesized reasons for the methane increase — both natural and anthropogenic — and compared them to the newly garnered ice core data. None of them alone proved sufficient for explaining the greenhouse gas increase. When he developed his own model combining characteristics of both the natural and anthropogenic hypotheses, it agreed closely with the ice core data.